1. Field of the Invention
The invention relates in general to a molding stack and is particularly, but not exclusively, applicable to a stack component used within a stack of an injection mold. In particular, it relates to a gate insert having a cooling channel surrounding the gate to improve heat transfer for cooling the mold.
2. Background Information
As is commonly known, a molding stack for injection molding a preform typically includes a core, a neck ring pair, a cavity, and a gate insert. The gate insert typically has a narrow cylindrical passage called a gate, through which a melt of thermoplastic material enters a cooled molding cavity. The time required to cool the melt of plastic contained in the gate, after the mold cavity has been filled and packed, often dictates the duration of the cooling phase of the molding cycle, and is a direct result of having started cooling last; and, due to the fact that the gate sits adjacent a heated hot runner nozzle (i.e. nozzle tip/insulator interface) and is therefore difficult to cool. This is particularly true of preform molds with extended gates. An extended gate produces a preform with an extended nub, the purpose of which is to encapsulate any imperfections in the nub. A portion of the nub may be trimmed in a post-mold process.
In the case of PET preforms, common manufacturing defects are:
Crystallinity: the resin recrystallizes due to the elevated temperature of the core resin not cooling quickly enough. The white appearance of the crystals impairs the clarity of the final product and provides an area of potential weakness in a resultant blown product, especially in the gate region.
Surface blemishes: the ejected preforms, initially having solidified surfaces, are reheated by the bulk/core material, which causes the surface to soften and be easily marked. The insulation properties of the plastic support, over time, the migration of heat to the surface of the preform to cause the surface reheating effect. Sometimes this surface reheating can be severe enough to cause touching parts to weld together.
Geometric inaccuracies: handling partly-cooled preforms or attempting to further cool them in devices that do not maintain their geometric shape while their surfaces are reheated can cause the preform's round diameter to become oval shaped or the smooth surface to become wrinkled or non-linear. Uniform cooling of the part is therefore important.
The above-noted problems can be alleviated somewhat by extending the cooling time of the injection molded preforms in their mold. However, this will cause the injection molding cycle to be lengthened, and is not desirable because cycle time increases and productivity is diminished.
One approach to overcome the aforementioned problems is to introduce a cooling structure to improve heat transfer and cooling of that part of the mold (i.e. the nub) in the gate.
This approach has been addressed in several different ways, each of which uses a cooling channel surrounding the gate.
However, in general terms, the prior art cooling channel configurations are regarded as having less than optimal heat transfer between the melt in the gate and the coolant in the cooling channel, due to excessive separation distance therebetween, that in turn dictates a longer cooling/cycle time. The excessive separation distance is often a result of having to provide the gate insert with a load bearing structure necessary to accommodate the high compressive mold clamping force that acts therethrough. The two most commonly known gate insert configurations include a cooling channel configuration that is either a circumferential groove or a diamond-drilling pattern.
The circumferential groove-cooling channel is typically formed from the outside surface of the gate insert adjacent the gate. The size of the cooling channel is quite limited in that it is desirable, from a flow dynamics perspective, to match the cumulative cross-sectional area of the flow paths around the gate to that of the source/sink coolant supply channels, provided through a cavity plate. Further, the maximum depth and overall profile of the groove is generally dictated by the configuration of the resultant web, between the groove and the gate, that must be capable of withstanding the applied mold clamping force without permanent deformation. In practice, the required size of the web dictates a relatively large separation distance between the cooling channel and the gate and as such does not provide for optimal gate cooling efficiency.
Alternatively, the diamond drilling cooling channel provides a crude approximation of a toroidal channel surrounding the gate, and is a resultant of an array of intersecting coplanar drill lines. The resultant flow channel is typically six-sided, as is considered to be the practical design limit. Again, the size of the cooling channel is chosen, from a flow dynamics perspective, to match the cumulative cross-sectional area of the flow paths around the gate to that of the source/sink coolant supply channels. Further, the crude form of the toroidal cooling channel dictates that the separation distance between the cooling channel and the gate varies along the flow path, and hence the optimal heat transfer occurs only at a limited number of points, six in the typical case. More particularly, non-uniform cooling can adversely affect port quality.
A further alternative is illustrated in DE 10024625, which proposes a copper alloy insert ring containing “diamond drilled” cooling channels. The channels result in non-linear heat transfer across the gate, which is undesirable. Furthermore, by choosing an insert ring an extra heat resistor is introduced due to the gap between stainless steel gate and copper alloy ring, thereby potentially reducing total heat transfer. The choice of copper may also critically impact the structural soundness of the gate when subjected to the high loads, in use.
The present invention and its preferred embodiments seek to overcome or at least mitigate the problems of the prior art.